In order to produce properly functioning semiconductor devices, tight dimensional control is typically required during nanofabrication. In processes such as growth or etching, control of feature size, especially height, is often performed by carrying out an initial calibration run on a dummy wafer, which includes a post-process measurement to determine the growth or etching rate. After this preliminary calibration, the fabrication is run on the real wafer under the same processing conditions, followed again by a post-processing measurement check. This iterative approach has obvious drawbacks including the added time and cost of duplicate runs, variation due to system drift, and lack of adaptive process control. Further, the characterization measurement often requires destruction of the sample. It is clear that accurate, non-destructive, real-time in-situ monitoring is highly desirable as it enables feedback and fine adjustment of the processing conditions.
In various fields, optical characterization methods fulfill the need for nondestructive testing. Examples include point measurement techniques such as spectroscopic ellipsometry, phase sensitive ellipsometry, laser reflectometry, multi-beam interferometry, and emission spectroscopy. In the case of semiconductor fabrication, it is typical practice to measure a structure height at a single point or region of interest and to infer information across the wafer on the basis of an assumption that the process is uniform. While this might be adequate for most planar processes, it is well known that the growth and etch rates in high-aspect-ratio structures, such as at the bottom of a trench, depend on the width of the opening. Consequently, methods based on imaging rather than on single-point measurements would be preferred.
Accordingly, imaging ellipsometry has recently been used to monitor the removal of large-area biomolecules during plasma etching. While the sensitivity of these single point or imaging methods can be a few nanometers; however, the measurements are typically hindered by vibrations and drift in the sample. Maintaining nanoscale accuracy in noisy environments is particularly difficult and represents the main challenge in developing real-time monitoring of dynamic fabrication processes such as wet etching.
During the past decade or so, techniques of quantitative phase imaging (QPI) have been developed for mapping an optical path length shift created by a sample at each point in an image. In particular, some QPI techniques may provide information at the nanoscale about the structure and dynamics of the specimen under investigation. One such QPI technique is diffraction phase microscopy (DQM), described in Popescu et al., Diffraction phase microscopy for quantifying cell structure and dynamics, Opt. Lett., vol. 31, pp. 775-77 (2006), incorporated herein by reference, which is both off-axis and common-path, thereby combining the benefits of fast acquisition rates and high temporal sensitivity.